Before we look at farming, lets save some starving dwarves, assuming you are in an environment where you can see trees growing on the surface, and it's safe to send dwarves up there. I am going to assume you did not embark on a mountain and nothing else, and likewise that you did not embark in a very rough environment (like a haunted or worse place), so your dwarves can often go onto the surface. Press 'd', then 'p', and press enter and drag your cursor over a large area filled with plants. The ones that are shrubs sometimes have fruit that can be eaten and/or brewed, and your dwarves (that have herbalist skill enabled at least) will go gather all the wild plants they can. Expect to try to gather from 10 shrubs before you get 1 plant - and that plant might not be edible. However, if you have dwarves that are almost dead, this is a way to try and feed them almost immediately.
Also before we look at farming, I know you have access to water. How about fishing? This is fairly fast and fairly safe, if your environment isn't to scary. Set a few dwarves to have the fishing labor enabled, and press 'b' 'w' 'h' to place a fishery workshop. Make sure at least one not very busy dwarf has fish cleaning as a job it can do. As soon as your dwarves catch some fish (don't worry if you cant see fish, they are often there and after a time the game will tell you that they are not if not) the fishery worker will start turning that fish into food (newly caught fish don't taste good without cleaning).
Hopefully your dwarves are not to hungry to cooperate, since maybe they've eaten from the instructions above. It's time to talk farms!
There's two types of seeds, two basic types at least.
One you embarked with, those are plump helmets among others. This type grows underground. Only underground.
They will grow in either 'dirt farms', that don't need to be watered ever, or on rock farms, that you cannot start until you water once.
To water something, you just want a little bit of water there. And then you want the water to go away. Really shallow water will evaporate. Many people make a way to get some water (you did that by channeling to your wall), but BEFORE they do that, they make a way to shut that water off so they can eventually get rid of it and enjoy the mud it made - This can be complex. I think more complicated then you are ready for, since the wiki is not working well for you.
So to start, you need to find DIRT that is SUBTERRANEAN in the DARK. You can use 'k' to do that, just move your cursor around with the arrow keys until you see something that looks like this:
It can be any kind of dirt, from sand to clay, but you don't want any sort of rock, not even mudstone or sandstone - if it's stone it has to be irrigated first. And the square has to be dark, and subterranean, or you will be trying to grow a totally different sort of plant.
Find a few squares of underground dirt that are dark, maybe a group of three by three (three tall by three wide) that form a square together. Press 'b', then 'p', then press 'u' twice, then 'k' twice. This will make a farm plot just that size and shape. Press enter when you have that superimposed over the dirt you picked out.
A dwarf with farming labor enabled will head over and build the farm plot. Now you've got more work to do, once that's done.
Press 'q' and move your cursor over the farm. It should look a lot like this - though with fewer plants - but you want to see plump helmets. If you don't, you have more problems, maybe you don't have any seeds for this sort of plant, or maybe you put the farm in the wrong place (like somewhere with light or maybe on rock). Press down arrow to highlight plump helmets, then enter.
Now press a, b, c, d, to change seasons that planting will happen, and select plump helmets for each one. There's several reasons why you don't want the other plants, but I recommend you experiment to see why - plump helmets for now will fix all your problems and fill all your needs, at least a little bit. Later you can do more complex things if you want.
Once you've done that, your dwarves should start running over with plump helmet seeds to plant, and in a few game weeks your troubles with food should be nearly over.
Above ground planting is not that much different, but you need to have the seeds of above ground plants, and you cant start with them. You'll find them through herbalism, trying to gather the wild plants. Once you eat them or brew them, there will be seeds, and you can grow those. You can also get aboveground seeds by trading, but that's a later thing too, and some seeds wont grow where you happened to embark, just like in life some plants need hotter/colder or wetter/drier conditions than you happened to pick this time. Above ground farming can be easier than below ground, but only you can tell if this is true for you. To make your above ground farm, do the same thing you did belowground, look for dirt but now you want to see Outside Light and Above Ground where you'd seen Inside, Dark, and Subterranean.
Good luck and keep having fun, this game is aweome and gets better the more you understand it!
You embarked somewhere, and the environment there changes how things work. If you can see trees and shrubs around the area you embarked in, you can farm there.